Modern communication channels need to carry various information including both voice and data. The data services typically include facsimile, e-mail, Internet access as well as other forms of data communication services. However, the bandwidth of the wireless spectrum is a limiting factor on the amount of information that can be transmitted. This makes it extremely important to maintain spectral efficiency.
Noise problems in wireless communication channels also cause various problems which limit the amount of information through these channels. Multipath causes the amplitude of the received channel to change. For a fixed noise at the receiver, multipath effects will actually change the signal to noise ratio/carrier to noise ratio.
Various techniques have been used to maximize the spectral efficiency of communications. Certain transmission techniques take advantage of the time-varying nature of wireless channels to vary the transmitted power level, symbol rate, coding rate and scheme and constellation size. These techniques are described in T. Ue, S. Sampei, and N. Morinaga, "Symbol rate and modulation level controlled adaptive modulation/TDMA/TDD for personal communication systems," in Proc. IEEE Veh. Technol. Conf. VTC'95, Chicago, Ill., pp. 306-310, July 1995. Full paper published in the IEICE Trans. Commun., vol. E78-B, pp. 1117-1124, August 1995; W. T. Webb and R. Steele, "Variable rate QAM for mobile radio," IEEE Trans. on Commun., vol. COM-43, pp. 2223-2230, July 1995; S. G. Chua and A. Goldsmith, "Variable-rate variable-power M-QAM for fading channels," in Proc. IEEE Veh. Technol. Conf. VTC'96, Atlanta, Ga., pp. 815-819, April 1996. Full paper to appear in the IEEE Trans. on Commun., October 1997; H. Matsuoka, S. Sampei, N. Morinaga, and Y. Kamio, "Adaptive modulation system with variable coding rate concatenated code for high quality multi-media communication systems," in Proc. IEEE Veh. Technol. Conf. VTC'96, Atlanta, GA, pp. 487-491, April 1996. Full paper published in the IEICE Trans. Commun., vol. E79-B, pp. 328-334, March 1996; M. S. Alouini and A. Goldsmith, "Adaptive M-QAM modulation over Nakagami fading channels," in To appear in Proc. IEEE Global Commun. Conf. GLOBECOM'97, Phoenix, Ariz., November 1997. None of these techniques use adaptive techniques of the type described in this application.
The goal of many of these techniques is to improve the average spectral efficiency. The spectral efficiency is often defined as the average transmitted data rate per unit bandwidth for an average specified carrier to noise ratio and bit error rate. Hence, there is often a trade-off between the noise on the channel and the amount of information which can be transmitted over the channel.
Moreover, accurate performance can be enhanced by accurate channel estimation at the receiver, and a reliable feedback path between that estimator and the transmitter. Such systems may also require buffering of the input data since assumption of a low carrier to noise ratio can cause a very high outage probability.